The catalytic epoxidation of an olefin by use of silver-based catalysts is well known in the art. However, conventional silver-based catalysts are often limited by their low selectivities. To a large extent, the selectivity determines the economical feasibility of an epoxidation process. For example, a one percent improvement in the selectivity of the epoxidation process can substantially reduce the yearly operating costs of a large-scale olefin oxide plant.
As is known to those skilled in the art, the produced epoxide is highly valuable. For example, the ethylene oxide may be reacted with water, an alcohol, or an amine to form ethylene glycol, a 1,2-diol ether, or an alkanolamine, respectively. Ethylene glycol, in turn, is used, for example, as a component in antifreeze compositions, or as a solvent, or a base material in the production of polyethylene terephthalates.
Highly selective silver-based epoxidation catalysts have been developed which extend the selectivity to a value that is closer to the stoichiometric limit. Such highly selective catalysts comprise a porous refractory support, such as alpha alumina, along with a catalytic amount of silver on its surface and at least one promoter that improves the catalyst performance in the epoxidation process.
The use of alkali and transition metals as promoters for silver catalysts is now common in the production of ethylene oxide by the partial oxidation of ethylene in the vapor phase. See, for example, U.S. Pat. Nos. 4,010,155, 4,012,425, 4,123,385, 4,066,575, 4,039,561 and 4,350,616. Highly selective catalysts which contain, in addition to silver, selectivity-enhancing promoters, such as rhenium, molybdenum, tungsten or nitrate- or nitrite-forming compounds, are discussed in U.S. Pat. Nos. 4,761,394 and 4,766,105. The catalyst may comprise further elements as described in U.S. Pat. Nos. 3,962,136 and 4,010,115.
Over the last two decades, rhenium has been shown to be effective in improving the selectivity of silver-based catalysts supported by a refractory porous support. See, for example, U.S. Pat. Nos. 4,761,394 and 4,833,261. Further improvement of silver-based catalysts promoted with rhenium has been achieved by the use of sulfur (S), molybdenum (Mo), tungsten (W), chromium (Cr) promoters, as disclosed in, for example, U.S. Pat. Nos. 4,766,105, 4,820,675 and 4,808,738.
In view of the above, there is a continuing need for silver-based epoxidation catalysts with improved performance, particularly in catalyst activity and selectivity.